1. Field of the Invention
This invention relates to optical passive components used in conjunction with a bi-directional amplifier system capable of simultaneously and independently providing optical amplification to light-wave data channels propagating in opposite directions through an optical fiber.
2. Description of the Related Art
The use of optical fiber for long-distance transmission of voice and/or data is now common. As the demand for data carrying capacity continues to increase, there is a continuing need to augment the amount of actual fiber-optic cable as well as to utilize the bandwidth of existing fiber-optic cable more efficiently. The latter practice of increasing the carrying capacity of existing fiber cable is sometimes referred to as the creation of xe2x80x9cvirtual fiberxe2x80x9d and is clearly more cost effective than adding real fiber.
One of the ways in which xe2x80x9cvirtualxe2x80x9d fiber is created is through the practice of Wavelength Division Multiplexing (WDM) in which multiple information channels are independently transmitted over the same fiber using multiple wavelengths of light. In this practice, each light-wave-propagated information channel corresponds to light within a specific wavelength range or xe2x80x9cband.xe2x80x9d To increase data carrying capacity in a given direction, the number of such channels or bands must be increased.
Additionally, it is desirable to use existing fiber for bi-directional communications. More particularly, through the use of WDM, a single optical fiber may be used to transmit, both simultaneously and independently, both eastbound (northbound) as well as westbound (southbound) data.
This bi-directional data-carrying capability of optical fiber increases the need for additional channels still further. However, since all the channels (wavelength bands) must reside within specific low-loss wavelength regions determined by the properties of existing optical fiber, increased channel capacity requires increased channel density. Thus, as the need for increased data carrying capacity escalates, the demands on WDM optical components-to transmit increasing numbers of more closely spaced channels with no interference or xe2x80x9ccrosstalkxe2x80x9d between them and over long distances-becomes more severe.
Optical amplifiers are important components of fiber-optic communication systems. Traditionally, signal regeneration has been accomplished through the use of repeaters, which are combinations of demultiplexers, receivers, signal recovery electronics, transmitters (light sources together with optical modulators), and multiplexers. In a repeater, the signal for each channel is recovered electronically and transmitted anew. Unfortunately, the complexity and cost of repeater-based systems becomes unwieldy with increase in the number of channels of WDM systems. Optical amplifier systems have therefore become attractive alternatives to repeaters. Erbium-doped fiber amplifier (EDFA) systems have become especially popular owing to their gain characteristics near the 1.5 xcexcm (micrometer) transmission band of conventional optical fiber.
EDFA systems are generally associated with sets of set of so-called xe2x80x9coptical passive componentsxe2x80x9d which perform various signal and laser pump beam combination, separation, and re-direction functions. Also included in the set of optical passive components are optical isolators, generally one disposed to either side of the optical gain element (Er-doped fiber), which guard against amplification and subsequent transmission of backward-propagating signals. An example of a conventional EDFA system 100 including the various optical passive components is shown in FIG. 1. More particularly, FIG. 1 shows an amplifier of the prior art, which includes a basic EDFA block diagram. The EDFA system 100 shown in FIG. 1 is a unidirectional amplifier.
In FIG. 1, optical isolators, such as isolator 101 and isolator 102, are disposed to either side of an Er-doped fiber 103. At least one pump laser, such as co-pump laser 104 and/or counter-pump laser 105, are generally required to generate the fluorescence in the Er-doped fiber 103, which leads to amplification. These pump lasers 104, 105 generally have wavelengths of 980 or 1480 nm whereas the signal(s) has a wavelength near 1500 nm. Generally, at least two lasers are used such that one laser-the co-pump laser 104-directs its light along the Er-doped fiber 103 in the same direction as signal propagation and the other laser-the counter-pump laser 105-directs its light in the direction opposite to signal propagation.
Because of the differences in wavelengths between signal(s) and laser(s), wavelength-division multiplexers/demultiplexers (WDM""s) such as input WDM 106 and output WDM 107 are required. Input WDM 106 combines the co-pump laser light together with the signal light such that both propagate in the same direction through the Er-doped fiber 103. Likewise, output WDM 107 combines the counter-pump laser light together with the signal light such that the two lights propagate in opposite directions through the Er-doped fiber 103. Furthermore, input WDM 106 and output WDM 107 remove any residual counter-pump light or co-pump light, respectively, from the system.
Other optical passive components, such as bandpass filter or isolator 108A and bandpass filter or isolator 108B, may be present to prevent laser light of the opposite laser from entering each respective pump laser. Finally, signal taps, such as input tap 109 and output tap 110, may be present so as to sample small proportions of the input and output signals, respectively. These samples of input and output signals may be directed to separate respective photo-detectors such as photo-detector 111A and photo-detector 111B so as to monitor the amplifier system performance using comparison and control logic 112.
Bi-directional lightwave communications systems are those in which signal lights are carried in both directions within individual optical fibers. In the current state of the art, separate amplifiers are used for eastbound (northbound) and westbound (southbound) communications channels as shown in the prior art band bi-directional amplifier 200 shown in FIG. 2. The counter-propagating signals are respectively separated and recombined on either side of the pair of optical amplifiers.
For instance, in FIG. 2, if the xe2x80x9cbluexe2x80x9d or relatively short wavelength band 201 shown as solid lines represents westward propagating signals and the xe2x80x9credxe2x80x9d or relatively long wavelength band 202 shown as dash-dot lines represents eastward propagating signals, then these two signals are separated and recombined by WDMs 203A and 203B. Between the two WDMs 203A, 203B, the blue and red signals propagate on separate physical optical fiber sub-paths 204 and 205, respectively, but to either side of each WDM, the westbound blue and eastbound red signals co-propagate along the same physical fiber pathways 211 and 212. Each of the fiber sub-paths 204 and 205 contains its own amplifier system, 206 and 207, respectively.
Optional second amplifiers 208 and 209 may be placed in each of the fiber sub-paths and the locations between each of the resulting sequential amplifiers 206 and 208 or 207 and 209 corresponds to mid-stage access ports 210A and 210B in the blue and red sub-paths, respectively. Generally, each of the optical amplifier systems, 206 and 207 and, optionally, 208 and 209, shown in FIG. 2, comprises all the basic components illustrated in FIG. 1 and possibly others. In particular, the amplifier 206 (and optionally 208) contains optical isolators that only permit westward light propagation and the amplifier 207 (and optionally 209) contains optical isolators that only permit eastward light propagation.
One example of the wavelength constitution of co-propagating bi-directional signals is illustrated in FIG. 3, which corresponds to a band bi-directional amplifier. In FIG. 3, as an example, the xe2x80x9cbluexe2x80x9d band 301 and the xe2x80x9credxe2x80x9d band 302 occupy separate wavelength regions each wholly contained within the well-known fiber transmission band 303 centered near a wavelength of 1.55 xcexcm. For instance band 301 might represent the wavelength constitution of the westbound signal channel(s) 201 of FIG. 2 while band 302 might represent the wavelength constitution of the eastbound signal channel(s) 202. This type of bi-directional lightwave transmission scheme is termed xe2x80x9cband bi-directionalxe2x80x9d transmission herein. Other types of band bi-directional transmission schemes are possible. For instance, the xe2x80x9cbluexe2x80x9d band might correspond to all or a portion of the 1.3 xcexcm fiber transmission band while the xe2x80x9credxe2x80x9d band might correspond to all or a portion of the 1.55 xcexcm transmission band, etc.
Optical amplifiers are costly and complex components of optical data and telecommunications systems. The prior-art bi-directional optical amplification system shown in FIG. 2 uses two such amplifiers, effectively doubling the cost, complexity, and bulk relative to unidirectional transmission systems. This doubling of systems is necessitated by the fact that optical isolators, which are integral passive components of optical amplifiers, generally perform isolation in a unidirectional sense, regardless of the wavelength of light propagated through them.
Accordingly, it is an object of the present invention to create such advantages, as described herein, over the prior art through the disclosure of a set of optical passive components that together comprise a bi-directional amplifier.
An object of the present invention is to provide a bi-directional optical amplification system of reduced complexity, cost, and bulk relative to prior art bi-directional amplification systems.
Another object of the present invention is to provide a bi-directional optical transmission system which includes a bi-directional optical amplifier.
A further object of the present invention is to provide a bi-directional optical transmission system providing selective, bi-directional isolation of optical signals.
Yet another object of the present invention is to provide a bi-directional polarization independent optical isolator simultaneously transmitting at least two separate signal rays in opposite forward directions while simultaneously suppressing backward transmission of each signal ray in its respective reverse direction, and a corresponding method thereof.
Still another object of the present invention is to provide a method of asymmetric interleaved bi-directional wavelength multiplexed optical signal propagation in a light wave communications system, wherein a first set of channels included within a first set of bands propagates in a first direction, a second set of channels included within a second set of bands propagates in a second direction opposite to the first direction, the width of bands of the first set of bands is not equal to the width of bands of the second set of bands, and the first and second bands are interleaved with one another.
To accomplish those objects, the present invention is a bi-directional polarization independent optical amplifier system simultaneously transmitting two separate signal rays in opposite forward directions while simultaneously suppressing backward transmission of each of the two separate signal rays in its respective reverse direction. The bi-directional polarization independent optical amplifier system of the present invention comprises a bi-directional polarization independent optical isolator of the present invention. The bi-directional polarization independent isolator of the present invention comprises a birefringent polarization element, a reciprocal rotation element, a non-reciprocal rotation element, a reflective element, and a lens.
The birefringent polarization element separates each of the two separate signal rays into components thereof upon a first traverse therethrough and selectively re-combines the components of the two separate signal rays into two respective output signal rays upon a second traverse therethrough. The reciprocal optical rotation element and the non-reciprocal optical rotation element together selectively rotate the direction of the plane of polarization of both of the components of each of the two separate signal rays depending upon the transmission direction of the each of the two signal rays. The reflective element reflects the components of each of the two separate signal rays and selectively rotates both of the components of one of the two separate signal rays. The lens collimates and directs the components of the two separate signal rays traveling in a forward direction onto the reflective element, and focuses the reflected components onto the birefringent polarization element.
The two separate signal rays are separated from each other based upon their respective wavelengths.
More particularly, the two separate signal rays are separated into two, respective and separate wavelength bands.
Alternatively, the two separate signal rays include two sets of wavelengths, each of the two sets of wavelengths including a plurality of wavelengths, such that wavelengths of the two signal rays are alternatingly interspersed with each other.
In addition, the present invention is a method of bi-directional optical amplification of signal rays within an optical communications system. The method of the present invention comprises simultaneously inputting into a bi-directional polarization independent optical isolator of the present invention of the optical communications system two separate signal rays of the optical communications system. The bi-directional polarization independent optical isolator of the present invention causes simultaneous transmission of the two separate signal rays in opposite forward directions through an optical gain element and simultaneously suppresses backward transmission of each of the two separate signal rays in its respective reverse direction.
Moreover, the present invention includes a method of asymmetric interleaved bi-directional wavelength multiplexed optical signal propagation in a light wave communications system, in which a first set of channels included within a first set of bands propagates in a first direction and a second set of channels included within a second set of bands propagates in a second direction opposite to the first direction, the width of bands of the first set of bands is not equal to the width of bands of the second set of bands, and the first and second bands are interleaved with one another.
These together with other objects and advantages which will be subsequently apparent, reside in the details of construction and operation as more fully hereinafter described and claimed, reference being had to the accompanying drawings forming a part hereof, wherein like numerals refer to like parts throughout.